Snap disc thermal sequencer

ABSTRACT

Apparatus for sequencing the switching of electrical loads comprises a plurality of switch cells built into an electrically insulating housing. Each cell includes a bimetallic disc, motion transfer pin, switch and terminal members. A disc plate is located adjacent one side of the discs to retain them in position while a control heater is positioned contiguous to the disc plate with suitable electrical insulation interposed therebetween. A heater grid cover and a terminal cover are located on opposite sides of the housing to complete the assembly. The control heater can take different forms including a printed heater on a ceramic substrate or a semiconductive steep sloped positive temperature coefficient of resistance (PTC) material and can be formed as individual heater elements on a common heat sink. Line voltage compensation, as by zener regulators, can also be provided.

il'nited States Patent Butler et a1.

[ 1 Jan. 23, 1973 SNAP DISC THERMAL SEQUENCER Inventors: Stuart L. Butler, Versailles, Ky.; Robert E. Crocker, Richardson, Tex.

[73] Assignee: Texas Instruments Incorporated,

Dallas, Tex.

Filed: Dec. 29, 1970 App]. No.: 102,472

[52] U.S. Cl. ..337/107, 219/486, 337/42, 337/95, 337/102, 337/104, 337/337, 337/339 Int. Cl. ..H0lh 71/16 Field of Search ..2l9/186, 508, 511; 337/42, 337/43, 95, 96,102,104,107,112, 337,

[5 6] References Cited UNITED STATES PATENTS ll/l970 1/1930 3/1965 FOREIGN PATENTS OR APPLICATIONS Berlin ..337/l12 X Bunkholder ..337/102 X Valverde ..337/102 X 2/1965 France ..337/338 1,487,654 7/1967 France ..337/ll2 Primary ExaminerBernard A. Gilheany Assistant Examiner-F. E. Bell Attorney-Harold Levine, Edward J. Connors, Jr., John A. Hau'g, James P. McAndrews and Gerald B. Epstein [57] ABSTRACT Apparatus for sequencing the switching of electrical loads comprises a plurality of switch cells built into an electrically insulating housing. Each cell includes a bimetallic disc, motion transfer pin, switch and terminal members. A disc plate is located adjacent one side of the discs to retain them in position while a control heater is positioned contiguous to the disc plate with suitable electrical insulation interposed therebetween. A heater grid cover and a terminal cover are located on opposite sides of the housing to complete the assembly. The control heater can take different forms including a printed heater on a ceramic substrate or a semiconductive steep sloped positive temperature coefficient of resistance (PTC) material and can be formed as individual heater elements on a common heat sink. Line voltage compensation, as by zener regulators, can also be provided.

13 Claims, 12 Drawing Figures PATENTEDJAH23 1975 3,713,062

Stuart L tZer, Robert firm'iz'er,

Inventors:

PATENTEDJAHNIBH 3.713.062

sum 3 [IF 4 Inventors:

Stuart L.,But2er, R0bert'. Cracker,

SNAP DISC THERMAL SEQUENCER This invention relates to a thermal sequencer to provide a time interval between the switching of electrical loads and more particularly to a sequencer with a plurality of bimetallic snap-acting discs which are caused to be operated following the energizing and deenergizing of a control heater. Among the several objects of this invention may be noted the provision of a thermal sequencer with a multiplicity of electrical circuits, with a minimum packaged size, with a minimum complement of components, with a simplicity in construction and economy in assembly. Another object is the provision of simplified external control circuitry by requiring one external circuit connection from the control heater of the thermal sequencer. Another object is to provide thermal sequencers with uniformly distributed control heaters and to be designed so that they may be easily duplicated in production. A further object is to provide a thermal sequencer'in which the number of sequencing stations can be easily varied and the time interval in operation between stations can be varied. Still another object is the limiting of the maximum temperature of the control heater.

Other objects and features will be in part apparent and in part pointed out hereinafter.

The invention accordingly comprises the elements and combination of elements, features of construction, arrangement of parts and planned sequence of operation which will be exemplified in the structures hereinafter described, and the scope of the application which will be indicated in the following claims.

In the accompanying drawings, in which several of the various possible embodiments of the invention are illustrated:

FIG. 1 is an enlarged plan view of a thermal sequencer, with the terminal cover broken away, according to the present invention.

FIG. 2 is a side elevation of FIG. 1.

FIG. 3 is a bottom view of FIG. 1 broken away to expose the various parts.

FIG. 4 is an enlarged elevation of the FIG. 1 embodiment taken along view line 44 of FIG. 1.

FIG. 5 is an enlarged horizontal cross-section along line 5-5 ofFIG. 1.

FIG. 6 is a perspective view of the disc plate.

FIG. 7 is a perspective view of a first embodiment of a heater employed in the FIG. 1 sequencer.

FIG. 8 is an enlarged perspective view of a heater grid cover shown broken away in FIG. 3.

FIG. 9 is an elevation of FIG. 8 partly broken away to show the grid structure.

FIG. 10 is a perspective view of a second embodiment of a heater.

FIG. 11 is a chart showing heating and cooling times for the switch cells of the thermal sequencer.

FIG. 12 is a partial schematic showing a modified means for limiting temperature of the heater.

FIG. 13 is a perspective view of a creep acting disc useful in the thermal sequencer of the instant invention.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings. Dimensions of certain of the parts as shown in the drawings may have been exaggerated or modified for the purposes of clarity of illustration.

trically insulative molded phenolic, is provided with cavities on one side generally indicated by numeral 16 as seen in FIGS. 1 and 5 and is adapted on the other side to support bimetallic discs. As seen in FIG. 5, an annular recessed surface is provided in the free distal end of molded cylindrical walls 18 to support within the housing snap-acting bimetallic discs 30 (removed from switch cell 4 and thus not shown), 32, 34, 36 and 38 respectively for switch cells 4, 6, 8, l0 and 12. A hub 19 is formed in the housing for each cell and contains a bore to slidably mount a motion transfer pin 50. Additionally, housing 14 contains six spaced posts 21 for mounting a heater assembly described below. The heater assembly comprises a disc retaining plate 22, a strip of electrical insulation 24, heater 26, and heater grid cover 28. On the side of housing 14 in which cavity 16 is located switch cell 6, as seen in FIG. 5, comprises tenninal 40 electrically connected by cantilever mounted electrically conductive flexible movable arm 42 with a movable electrical contact 44 mounted thereon attached by means such as rivet 45 to housing 14 and additionally terminal 46 which is made of relatively stiff electrically conductive material, has mounted thereon, as by welding, stationary electrical contact 48 is attached to housing 14 by conventional means such as rivet 49. Terminal cover 20, formed of a strip of electrical insulation and provided with terminal receiving apertures, may conveniently be placed on the housing to enclose the switch cells. Adjusting screw 47 located in tapped hole 51 of housing 14 bears against that portion of stationary terminal 46 which mounts stationary contact 48 and serves as a calibrating screw to adjustably locate contact 48. Contact 44 is located generally opposite contact 48 and is movable into and out of engagementtherewith. Flexible arm 42 is biased so that in its at rest position contact 44 engages contact 48 which is located as desired by means of adjusting screw 47 to complete an electrical circuit from terminal 40 through flexible arm 42, movable contact 44 to stationary contact 48 and terminal 46. Motion transfer pin 50 which is slidably mounted in a bore in hub 19 for sliding motion in opposite directions is chosen in a length for proper operation of contacts 44 and 48 in relation to the configuration of snap-acting bimetallic disc 32. In FIG. 5 disc 32 is shown in its low temperature configuration or position and is retained within support 18 of housing 14 by disc plate 22 made of metal such as stainless steel. Plate 22 contains apertures 23 as seen in FIG. 6, one for each disc, which are smaller in diameter than the bimetallic discs. In the low temperature configuration the discs bear against motion transfer pin 50 at end 54 so that it bears against flexible arm 42 at end 56 to maintain contacts 44 and 48 out of engagement. When disc 32 is at its high temperature position, its configuration or curvature is reversed from that shown in FIG. 5 permitting pin 50 to slide in hub l9'from the force transmitted by biased flexible contact arm 42 so that contacts 44 and 48 engage in electrical contact.

Switch cells 8, and 12 are similar to switch cell 6 as described.

Switch cell 4 is also similar to cell 6 except that the low temperature configuration or position of snap-acting bimetallic disc 30 is opposite to the position shown for disc 32 FIG. 5 so that the contacts in the switch of cell 4 are engaged electrically. Terminal 86 of cell 4, shown in FIG. 1, corresponds to terminal 40 of cell 6 and terminal 85, since it is internally connected to heater 26 is cutoff near the surface of housing 14, corresponds to terminal 46. Thus, with snap-acting disc 30 (not shown) in its low temperature position, an electrical circuit is provided from terminal 86 through the switch of cell 4 to cut off terminal 85. When snap-acting disc 30 is in its high temperature position, the electrical circuit between terminals 86 and 85 is opened. Perhaps it should be noted that construction can be made such that any or all cells can be so constructed as to open electrical circuits on temperature rise or any combination of circuit opening or closing on temperature rise.

Heater 26, best seen in FIG. 7, comprises a ceramic substrate 57 such as alumina on which is deposited, such as by silk screen printing, suitable conductive ink to form electrical resistance heater surface 58 and electrically conductive silver or other suitable material to form contact stripes 60 and 62. Insulation 24 seen in FIG. 2, comprises a strip of electrically insulating material such as fluorinated ethylene propylene film or polyimide film which serves to space apart and electrically insulate heater 26 from disc plate 22. Heater 26 is assembled in sequencer 2 so that heater surface 58 contacts disc plate 22 through insulation 24 which prevents shorting out of the heater surface and provides thermal coupling between heater 26 and plate 22. Additionally, apertures 23 provide windows in disc plate 22 for heater surface 58 to be radiantly exposed to snap-acting bimetallic discs 30, 32, 34, 36 and 38 through insulation 24, so that the discs are heated in part by conduction and in part by radiation from heater surface 58.

Heater grid cover 28, which may be made of conventional electrically insulative molded phenolic is shown in FIGS. 8 and 9 to contain a recessed seating portion or ledge 64 which is adapted to receive and locate heater 26. Grids 66 define heater apertures 68 exposing the surface of the substrate 57 of heater 26 to enhance cooling.

Heater contact 72 shown in FIG. 4 is made of an electrically conductive spring material such as phosphor bronze with free distal end 74 formed to facilitate a sliding electrical contact and its opposite end formed to facilitate attachment to housing 14.

Electrical connection to one end of heater 26 at contact stripe 60 is provided through aperture 70 in disc plate 22 (see FIG. 6) and a similar aperture in insula' tion 24, (seen in FIG. 4) by free distal end 74 of heater contact 72 which electrically engages contact stripe 60. The other end of heater contact 72 is attached to housing 14 by rivet 76. Heater terminal 78, formed of electrically conductive material, is also conveniently attached to housing 14 by rivet 76. This provides an electrical connection from terminal 78 through rivet 76 to heater contact 72 on to contact stripe 60 which is in electrical connection with one end of electrical resistance heater surface 58. The opposite end of electrical resistance heater surface 58 is in electrical connection with contact stripe 62 (seen in FIG. 7). Electrical connections to stripe 62 are made in a similar manner by a second heater contact 80, see FIGS. 1 and 3. End 82 of contact bears against stripe 62 and is aligned in a direction along the length of housing 14 compared to heater contact 72 aligned in a direction along the width of housing 14. The opposite end of heater contact 80 is attached to housing 14 by rivet 84 which also fastens cutoff terminal 85 to housing 14. The electrical circuit for heater 26 thus is from heater terminal 78 through rivet 76, heater contact 72 which contacts heater stripe 60 on through the heater resistance layer 58, heater stripe 62 which is contacted by heater contact 80 to rivet 84, cutoff terminal 85 and finally continuing through the switch of cell 4 to heater terminal 86.

Switch cells 8, l0 and 12 which are similar to cell 6 can readily be omitted or included as desired to vary the number of cells in the sequencer. Also housing 14 can be increased in size by providing additional cavities 16, supports 18 and hubs 19 for switch cells in addition to those shown in FIGS.'1 and 3.

As described above, heater 26 in FIG. 7 is monolithically printed or deposited on a single substrate. This achieves uniformity in characteristics across the surface area 58 and provides a single heater element for a plurality of switch cells.

Operation of thermal sequencer 2 will now be described with reference to FIG. 11. Heating curve represents the temperature of snap-acting bimetallic disc 30 versus time after applying voltage heater terminals 78 and 86 energizing heater 26 starting from an ambient temperature, Ta. The temperature of bimetallic snap-acting discs 32, 34, 36 and 38 are also represented closely by curve 120 since the construction shown of sequencer 2 is symmetrical and the heating is uniform across the heater surface 58 of heater 26. Cooling curve 122 represents the temperature of bimetallic disc 30 and closely that of discs 32, 34, 36, 38 versus time after heater 26 is deenergized starting from temperature T7. T1, T2, T3, T4 and T5 represent the temperatures at which bimetallic discs 38, 36, 34, 32 and 30 respectively snap to their high temperature position and T6, T8, T9, T10 and T11 are the respective temperatures at which the discs snap to their low temperature position.

On applying power to heater terminals 78 and 86 which energizes heater 26, when bimetallic discs of switch cells 4, 6, 8, 10 and 12 are at temperature Ta, switch cell 12 operates to close its contacts after time delay t1, switch cell 10 closes its contacts after time t2 for an additional delay after cell 12 of :2 less 11 and similarly for times t3, t3 less 22, t4 and :4 less :3 for switch cells 8 and 6. Heating continues until time 15 when high temperature snapping point T5 of bimetallic disc 30 is reached and switch cell 4 operates to open its contacts deenergizing heater 26. The heater cools until disc 30 reaches its low temperature snapping point T6 at which time switch cell 4 will close its contacts reenergizing the heater. Disc 30 temperature will cycle between T5 and T6 as long as power is supplied to heater terminals 78 and 86.

On removing power from heater terminals 78 and 86, when bimetallic discs are at temperature T7, bimetallic disc 30 cools to its low temperature snapping point T6 at which point bimetallic disc 30 resets. As cooling continues, bimetallic disc 32 reaches its low temperature snapping point T8 opening the contacts of switch cell 6 after time delay t8; on further cooling to the low temperature snapping point of disc 34, switch cell 8 opens its contacts after time delay 1.9 or an additional delay after cell 6 of 29 less t8 and similarly for times :10, H0 less t9, tll and tll less 10 for cells 10 and 12.

One of the uses which sequencer 2 is particularly well suited is in conjunction with forced air electric heaters or furnaces. In such an application, operating requirements would typically call for the fan motor and the first bank of heater elements to be turned on approximately simultaneously after an initial time delay and the remaining banks of elements to be turned on with a time delay of approximately 10 seconds between stages. Assuming the furnace comprises three banks of heater elements and a fan motor, switch cells 10, 8 and 6 would be used for the banks of heater elements, switch cell 12 for the fan motor and switch cell 4 to control the maximum temperature of heater 26 of sequencer 2. Typical temperature settings for the bimetallic discs would be as follows:

Disc Snapping Point Switch Bimetallic High Low cell no. Disc No. Temperature Temperature 4 30 245F. 225F.

Heater 26 is designed to provide a rate of temperature rise which results in operation of switch cells 12 and 10 typically in approximately 30 seconds after the furnace control is turned on, which applies power to heater terminals 78 and 87, to energize the first bank of heater elements and the fan motor; in approximately 10 second intervals the second and third banks of heater elements are energized as switch cells 8 and 6 operate and switch cell 4 operates subsequently in a cyclic mode as determined by the high and low temperature snapping points of disc 30. For shut-down, the room thermostat or similar sensor removes power from heater terminals 78 and 86 which permits heater 26 to cool resulting in switch cell 6 operating to turn off the third bank of heater elements which is that bank last energized. Approximately seconds later, switch cell 8 operates turning off the second bank of heater elements and, in approximately an additional 15 seconds, switch cells 10 and 12 operate to turn off the first bank of elements and the fan motor. The functions of cells 12 and 10 can be combined by using a single disc (combining discs 38 and 36) to operate a double pin 50 to operate two sets of switch arms and thereby control two electrical circuits with the operation of one disc. Economies and timing advantages are apparent. 4

It should be noted that heater 26 may be formed by printing the resistive coating 58 on the reverse side of substrate 57 thereby eliminating the need of insulation 24 and, if desired, disc retaining plate 22.

Another embodiment of the invention is shown in FIG. 10 in which heater 90 comprises a substrate 95 such as alumina on which is deposited conductive inks to form electrical resistance surface areas 96, 98, 100,

102 and 104 which include supplemental areas 106, 108, 110, 112 and 114 respectively. Electrically conductive silver is deposited in a continuous layer to form contact pad 92 and its stripe running across the center of the heater contacting one side of each electrical resistance surface area. A second continuous layer of conductive silver is deposited to form contact pad 94 and its associated continuous stripes contacting the opposite side of each resistance surface area. Thus all electrical resistance surface areas are connected in parallel configuration with contact pads 92 and 94. In this embodiment, each electrical resistance surface area is individually trimmed by removing portions of supplemental areas 106, 108, 110, 112 and 114 to make all surface areas alike or different in characteristics as desired. Heater 90 is assembled into thermal sequencer 2 in a similar manner to heater 26 of FIG. 1 since contact pad 92 corresponds to contact stripe 60 and pad 94 to stripe 62 for contacting heater contacts 72 and respectively. Resistance surface areas 96, 98, .100, 102 and 104 provide individual heaters for switch cells 12, 10, 8, 6 and 4 respectively on a shared common heat sink. Simultaneous energizing of all heater surface areas is achieved through a single pair of electrical connections avoiding additional external circuitry. Although the electrical resistance surface areas are shown connected in parallel to contact pads and associated stripes 92 and .94, it is within the purview of this invention for the resistance surface areas to be connected in series or series-parallel combination with pads 92 and 94 to obtain particular desired heater characteristics such as wattage, voltage, surface area and rate of temperature rise.

Heater 90, when trimmed so that heater resistance surfaces areas 96, 98, 100, 102 and 104 are all approximately the same in characteristics, such as resistance and temperature rise, provides equivalent operation to that heretofore described with heater 26 when substituted for it.

Increased flexibility in design of sequencer 2 is obtained where resistance surface areas 96, 98, 100, 102 and 104 of heater are trimmed to different characteristics of resistance and temperature rise. Turn on time delay in operation of the switch cells is determined by the snapping temperature point and rate of temperature rise of the bimetallic discs. For example, a sequencer incorporating a set of bimetallic discs having a set of snapping temperature points tabulated above with reference to heater 26, and with resistance surface areas 96 and 98 of heater 90 trimmed to provide an increase in temperature rise, would have a decrease in time for operation of switch cells 12 and 10, below the 30 seconds previously indicated for turn on of the fan motor and the first bank of heater elements while the additional time delay for operating switch cells 8 and 6 for turning on the second and third banks of heater elements would remain the same at approximately ten second intervals. An increase or decrease in the additional time delay for switch cells 8 and 6 would be obtained by trimming heater surface areas 100 and 102 respectively, depending upon the extent of removal of heater trim areas and 112, to higher or lower ohmic resistance values for lower or higher respective temperature rise characteristics.

An additional advantage the FIG. 10 embodiment offers is the opportunity to use bimetallic discs with approximately the same snapping temperature point in all the switch cells and obtain the desired time delay in operation between cells by trimming the heater resistance surface areas to the desired characteristics. Cooling time delay for turn off is adjusted as desired by selection of low temperature snapping points of the bimetallic discs.

Switch cell 4, which is electrically in series with the circuit of heater 26, can be varied from the construction previously described within the purview of the invention. One modification would be the replacement of snap-acting bimetallic disc 30 with a creep action bimetallic disc 30 as shown in FIG. 13. Another variation would be the employment of a bimetallic strip as the thermal element in place of bimetallic disc 30. Yet another variation would be the accommodation of a bimetallic strip with a movable contact attached, to replace disc 30 and motion transfer pin 50, which would electrically engage a suitably located stationary contact and terminal replacing contact 48 and terminal 46. As shown in FIG. 12, a further change would be the use of element 120 composed of a steep sloped positive temperature coefficient of resistance (PTC) material in series connection with heater 26 for sensing and limiting the heater temperature. Element 120 is conveniently located in switch cell 4 in place of its associated components. Also when it is desired to limit variation in line voltage across heater 26 so that variation in rate of rise in temperature of the heater is correspondingly limited, a zener diode regulator may be incorporated within switch cell 4. Drawings are not included for all of the various modifications mentioned since the changes are within the skill of the art.

As many changes could be made on the above constructions, such as incorporating individual self contained switch cells in housing 14, without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense, and it is also intended that the appended claims shall cover all such equivalent variations as come within the true spirit and scope of the invention.

It is to be understood that the invention is not limited to its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practiced or carried out in various ways.

What is claimed is:

l. A thermal sequencer comprising an electrically insulative housing containing a plurality of switch cells; each cell comprises an electrical switch, switch terminals, a snap-acting disc, and motion transfer means which extends between the snap-acting disc and the electrical switch; an electrical resistance heater in heat transfer relation with each of the snap-acting discs; a metallic disc retaining plate located adjacent the discs to retain them in the housing; a layer of insulation interposed between the heater and the disc plate to electrically insulate the heater from the metallic disc plate; a heater cover member mounting the heater in a recess therein and formed with a grid construction which supports the heater and which also provides apertures in the heater cover member; and an electrically insulative cover having apertures through which the switch terminals extend, the cover providing electrical insulation among the terminals while permitting access to them.

2. A thermal sequencer as set forth in claim 1 wherein the snap-acting discs have high and low temperature snapping points such that the time interval between respective electrical switches will be approximately the same.

3. A thermal sequencer as set forth in claim 1 wherein the snap-acting discs have high and low temperature snapping points such that the time interval between respective electrical switches will be different.

4. A thermal sequencer as set forth in claim 1 wherein said electrical resistance heater is deposited in one continuous layer on an electrically insulative substrate.

5. A thermal sequencer as set forth in claim 1 wherein said electrical resistance heater is deposited in separate areas on an insulative substrate whereby each heater area may be individually trimmed for each switch cell and means is provided for connecting the heater areas for electrical energization.

6. A thermal sequencer as set forth in claim 1 including means to electrically connect one of the electrical switches in series with the electrical resistance heater to serve to regulate maximum temperature of the heater.

7. A thermal sequencer as set forth in claim 1 including another cell and electric switch therein and means to electrically connect said another electrical switche in series with the electrical resistance heater, the cell containing the series connected switch also containing a creep action bimetallic thermal element which serves to regulate the maximum temperature of the heater.

8. A thermal sequencer as set forth in claim 1 further comprising a thermal element of steep sloped positive temperature coefficient (PTC) material and means to serially connect the material to the electrical resistance heater to limit the maximum temperature thereof to a predetermined value.

9. A thermal sequencer as set forth in claim 1 wherein said electrical resistance heater is deposited in a plurality of separated areas on an electrically insulative substrate and each heater area is trimmed to a preferred characteristic for each respective switch cell and means electrically interconnecting the plurality of heater areas on the substrate so that they are energized by connection to a single source of power.

10. Electrical sequencing apparatus comprising:

a housing of electrically insulative material, the housing having a first side formed with a plurality of switch cavities and a second side formed with a plurality of depending tubular walls defining a disc mounting area, one for each switch cavity and aligned therewith, a hub separating each switch cavity from each respective disc mounting area, each tubular wall having a free distal end, an annular groove formed in the free distal end of each tubular wall, a snap-acting disc mounted in each annular groove, a switch including a movable contact arm mounted in each switch cavity, a bore provided in each hub, slidable motion transfer means slidably mounted in each bore and adapted to transfer motion from each disc to the respective movable contact arm;

disc retaining plate provided with a plurality of apertures, each aperture aligned with a respective disc, each aperture being slightly smaller than each respective disc, the plate retaining the discs in their respective annular grooves; and

heater assembly mounted adjacent the discs and disc retaining plate, the assembly comprising an electrically insulative substrate, an electrically resistive coating located on the substrate, spaced electrically conductive contact layers on the resistive coating, means to electrically connect the heater for energization, a heater cover member having a recessed portion mounting the substrate, grid members defining apertures in the heater cover member in communication with the recessed portion, the heater cover mounted on the housing so that the resistive coating is contiguous to and is facing the discs, and a layer of electrically insulative material interposed between the resistive coating and the discs and disc retaining plate.

11. Apparatus according to claim 10 including means to serially connect one of the switches to the electrically resistive coating and the discs are chosen to snap at different temperatures to effect a sequential operation of the plurality of switches.

12. Apparatus according to claim 10 in which the resistive coating is formed in separate areas, each area located in heat transfer relation with a respective disc.

13. Apparatus according to claim 12 in which the discs are chosen to snap at approximately the same temperature. 

1. A thermal sequencer comprising an electrically insulative housing containing a plurality of switch cells; each cell comprises an electrical switch, switch terminals, a snap-acting disc, and motion transfer means which extends between the snapacting disc and the electrical switch; an electrical resistance heater in heat transfer relation with each of the snap-acting discs; a metallic disc retaining plate located adjacent the discs to retain them in the housing; a layer of insulation interposed between the heater and the disc plate to electrically insulate the heater from the metallic disc plate; a heater cover member mounting the heater in a recess therein and formed with a grid construction which supports the heater and which also provides apertures in the heater cover member; and an electrically insulative cover having apertures through which the switch terminals extend, the cover providing electrical insulation among the terminals while permitting access to them.
 2. A thermal sequencer as set forth in claim 1 wherein the snap-acting discs have high and low temperature snapping points such that the time interval between respective electrical switches will be approximately the same.
 3. A thermal sequencer as set forth in claim 1 wherein the snap-acting discs have high and low temperature snapping points such that the time interval between respective electrical switches will be different.
 4. A thermal sequencer as set forth in claim 1 wherein said electrical resistance heater is deposited in one continuous layer on an electrically insulative substrate.
 5. A thermal sequencer as set forth in claim 1 wherein said electrical resistance heater is deposited in separate areas on an insulative substrate whereby each heater area may be individually trimmed for each switch cell and means is provided for connecting the heater areas for electrical energization.
 6. A thermal sequencer as set forth in claim 1 including means to electrically connect one of the electrical switches in series with the electrical resistance heater to serve to regulate maximum temperature of the heater.
 7. A thermal sequencer as set forth in claim 1 including another cell and electric switch therein and means to electrically connect said another electrical switche in series with the electrical resistance heater, the cell containing the series connected switch also containing a creep action bimetallic thermal element which serves to regulate the maximum temperature of the heater.
 8. A thermal sequencer as set forth in claim 1 further comprising a thermal element of steep sloped positive temperature coefficient (PTC) material and means to serially connect the material to the electrical resistance heater to limit the maximum temperature thereof to a predetermined value.
 9. A thermal sequencer as set forth in claim 1 wherein said electrical resistance heater is deposited in a plurality of separated areas on an electrically insulative substrate and each heater area is trimmed to a preferred characteristic for each respective switch cell and means electrically interconnecting the plurality of heater areas on the substrate so that they are energized by connection to a single source of power.
 10. Electrical sequencing apparatus comprising: a housing of electrically insulative material, the housing having a first side formed with a plurality of switch cavities and a second side formed with a plurality of depending tubular walls defining a disc mounting area, one for each switch cavity and aligned therewith, a hub separating each switch cavity from each respective disc mounting area, each tubular wall having a free distal end, an annular groove formed in the free distal end of each tubular wall, a snap-acting disc mounted in each annular groove, a switch including a movable contact arm mounted in each switch cavity, a bore provided in each hub, slidable motion transfer means slidably mounted in each bore and adapted to transfer motion from each disc to the respective movable contact arm; a disc retaining plate provided with a plurality of apertures, each aperture aligned with a respective disc, each aperture being slightly smaller than each respective disc, the plate retaining the discs in their respective annular grooves; and a heater assembly mounted adjacent the discs and disc retaining plate, the assembly comprising an electrically insulative substrate, an electrically resistive coating located on the substrate, spaced electrically conductive contact layers on the resistive coating, means to electrically connect the heater for energization, a heater cover member having a recessed portion mounting the substrate, grid members defining apertures in the heater cover member in communication with the recessed portion, the heater cover mounted on the housing so that the resistive coating is contiguous to and is facing the discs, and a layer of electrically insulative material interposed between the resistive coating and the discs and disc retaining plate.
 11. Apparatus according to claim 10 including means to serially connect one of the switches to the electrically resistive coating and the discs are chosen to snap at different temperatures to effect a sequential operation of the plurality of switches.
 12. Apparatus according to claim 10 in which the resistive coating is formed in separate areas, each area located in heat transfer relation with a respective disc.
 13. Apparatus according to claim 12 in which the discs are chosen to snap at approximately the same temperature. 